1. Field of the Invention
The present invention relates to a thermal line printer of the type having a divided line head composed of a plurality of linear thermal head segments or "tiles" aligned in the width direction of a recording medium sheet perpendicular to the feeding direction or the lengthwise direction of the recording medium sheet so that a predetermined number of thermal print elements of adjacent tiles overlap so as to cover the entire span or width of the recording medium sheet, and particularly to a system for distributing print data bits among the overlapping print elements.
2. Description of the Prior Art
Typical thermal line print heads are constructed with a linear array of thermal print elements that are individually heated by data encoded energization signals to effect by sublimation the transfer of a dye donor onto the pixel or dot area line by line onto a moving a receiver sheet to form a two-dimensional image thereon. Such printers are described, for example, in commonly assigned U.S. Pat. Nos. 4,745,413 to Brownstein et al. and 4,710,783 to Caine et al., both of which are incorporated herein by reference. In such printers, the linear array segments are typically constructed to extend about 12 inches in width and are capable of printing lines of image pixels across typical 11 inch (or less) printing paper. The dye-donor film patches are similarly dimensioned so that it is not possible to print color images of a single image frame exceeding the widths of the linear array and the dye donor patch. The dye donor patch is also limited in length.
In order to overcome the length limitation of the dye donor patches, commonly assigned U.S. Pat. Nos. 5,132,701 and 5,140,341 to Stephenson and Fiscella, both incorporated herein by reference, describe a system and method for successively printing each of the three, single color separation, two-dimensional images in superimposition on a receiver to form a complete multi-color image having a length exceeding the comparable length of the dye-donor patches. In the '701 and '341 patents, two (or more) dye donor patches of the same color are employed in the printing of each of the color separation images in an image region on the receiver sheet. The data set for the entire, two-dimensional, color separation image is divided into first and second sub-image data sets for respective first and second sub-images in the length dimension, wherein the sub-images overlap one another in an overlap region comprising a certain number of print lines. The corresponding overlap line print data is distributed in a complementary manner in the first and second sub-image data sets with a blank print data bit pattern to mask any straight line effect at the boundary of the two sub-images that might otherwise appear on the receiver.
The sub-image data sets are successively applied to the thermal print head elements during the advancement of the receiver and the two, same color, dye donor patches past the print head array. The print head responds by successively printing the two sub-images in a merged fashion in the overlap region. Thus, it is possible using these techniques to print elongated image formats as long as the image format does not exceed the nominal 12 inch width of the single thermal head array.
However, it remains desirable to make larger prints despite the costs inherent in the gravure printing of dye-donor patches exceeding the nominal 12 inch width dimension. In order to print images which exceed the nominal 12 inch width in both dimensions, it has been proposed to construct a composite print head that is composed of a plurality of linear thermal print head segments or tiles, each consisting of a nominal 12 inch array of thermal print elements, aligned end to end in a single row and coupled to a source of data bits for selectively energizing the print elements in order to print images on large size recording paper such as A1 size and A0 size. Such an arrangement of series-connected, linear print head segments is more economic to manufacture, given the poor manufacturing yield rate of corresponding monolithic thermal print heads of comparative length.
However, this type of conventional elongated, multi-segment, thermal print head suffers the drawback that the pitch of the individual heating elements is irregular along the junction or connecting portion of adjacent thermal print head segments and thereby causes lines of higher (if too close together) or lower (if too far apart) density to appear at the juncture of each print head segment and thus impairs the quality of the printed image pattern.
The prior art discloses several methods for creating such elongated, multi-segment, thermal print heads. One technique is to fabricate the linear print head segments with the thermal elements disposed up to the edge of each segment. These segments are then bonded end to end onto a carrier plate so that the edge elements are in close proximity, and the segments form a single, linear array of thermal elements. When these heads are manufactured, there may be a discontinuity and minor misalignment between the edge elements that results in a line or streak at the transition elements during printing. These heads are useful in applications which require only black or white marking, or in applications which do not require high quality. The density discontinuity is more noticeable in applications that require smooth, continuous tone, such as near photographic quality prints.
A further type of conventional, elongated, multi-segment, thermal print head is the "divided line" head, disclosed, for example, in U.S. Pat. Nos. 4,660,052 to Kaiya et al., 4,977,410 and 5,003,323 to Onuki et al., and 5,119,108 to Hatakeyama. The divided line thermal print head is composed of a plurality of linear thermal head segments aligned in first and second of parallel rows in staggered relation and in partially overlapping relation at end portions of each adjacent linear segment in the sheet feeding direction so as to completely cover the entire width of recording paper. In operation each line of data bits to be printed is divided between first and second line data sub-sets for the respective first and second parallel rows. The first, or upstream, row of linear thermal element segments is activated with the first line data sub-set to print a part of the single line. After the receiver is advanced the requisite distance, the second, or downstream, row of linear thermal element segments is energized with the second line data sub-set to thereby complete the single line printing.
In such operation, in order to avoid duplicate printing by the overlapping portion of the staggered segments between the first and second rows, a predetermined number of thermal printing elements in each overlapping set are blanked during the printing operation with the first and second line data sub-sets. The staggered linear print head segments must be precisely positioned relative to each other to set a predetermined overlapping dimension corresponding to the span of the predetermined number of the blanked heating elements. Again, it is practically quite difficult to precisely and equally set the overlapping dimension of the individual staggered segments between the adjacent pair of rows during assembly of the divided line head and to maintain the set overlapping dimension due to thermal expansion of the linear segments during the continuous printing operation.
The '410, '323 and '108 patents are directed to software and hardware based systems for controlling the selection of the blanked overlapping elements of the linear head segments. In these patents, the system electrically defines the end thermal print elements of the upstream and downstream alternate line segments to minimize the line memory buffer space and to compensate for the manufacturing error offset between adjacent line segments forming the two displaced lines of segments.